Grimaces
Grimaces are invulnerable stone heads which attack Isaac until all other enemies in the room have been killed.
This page is about Grimaces added in Fiend Folio. More information about Grimaces in general can be found in the vanilla page.
Frowny
Frownies are smaller variants of Grimaces. Due to their small size, they are not attached to the ground and can be pushed around when walked into or by Isaac's tears. They fire small, short-ranged projectiles more frequently than Stone Grimaces. They will die when the room is cleared instead of deactivating.
Wetstone
Wetstones are Grimaces found in the Flooded Caves. They continuously spawn large amounts of 
Bubbles around themselves randomly which drift and bounce around the room, but can be destroyed by Isaac.
Super Grimace
Super Grimaces are large versions of Grimaces. They fire at a much slower rate, but when in range of Isaac, they will charge up for a moment to fire a large projectile towards him. This projectile travels a set distance before stopping and hovering in place, and then bursting into 4 smaller projectiles in the cardinal directions. It will also release these projectiles if it hits a wall or obstacle.
Furnace
Furnaces are encountered in the Dank Depths, and will periodically heat up all black creep in the room, causing multiple small bubbles to form within it. These bubbles will eventually burst into a small burst of arcing tar projectiles, but Isaac can step on them to destroy them harmlessly.
Sensory Grimace
Sensory Grimaces face one of the cardinal directions and rapidly shoot a stream of projectiles whenever Isaac moves in the same direction as the arrow shown on their head.
Like 
Constant Stone Shooters, some Sensory Grimaces deactivate on room clear while others can remain active in empty rooms; this is determined by the room layout.
Immural
Immurals hang from the ceiling by a cord and attach a cord between themselves and all other nearby enemies. This will prevent attached enemies from moving outside the range of the Immural. If all enemies attached to the Immural are killed, it will fall to the ground and scatter several projectiles around itself. Afterwards, it will remain half-buried in the ground, will block Isaac's projectiles and movement, and remains in that state even after the room is cleared and re-entered.
Immurals can spawn attached to invulnerable enemies such as other Grimaces. In this case they will not fall after the room is cleared as long as the invulnerable enemy persists.
Immurals will only attach themselves to enemies that spawned within their radius at the start of the room — with a range roughly 2 tiles in radius.
Cauldron
Cauldrons will periodically mark a random enemy in the room with a purple symbol above their head before firing a purple beam at it, creating a duplicate copy of it. The more health an enemy has, the longer it takes for a Cauldron to duplicate it.
The double will always have the same amount of health as the original at the time of duplication, copying over any damage taken.
Notes
- Cauldrons cannot duplicate
Psi Hunters.
Trivia
- In the mod's files, Furnace is called "Cooker".
- Frowny and Super Grimace have unique appearances when encountered on the Tomb floor if the Revelations MOD is installed.
Gallery
An activated Furnace.
An activated Sensory Grimace.
Other Sensory Grimaces with different marked arrows.
An Immural fallen into the ground.
Immural's appearance in Corpse.
An Immural fallen into the ground (Chest).
An Immural fallen into the ground (Dark Room).
An activated Cauldron.
Frowny's appearance in Tomb in the Revelations MOD.
Super Grimace's appearance in Tomb in the Revelations MOD.
Old version of Cauldron.
